(i) Field of the Invention
The present invention relates generally to power transformer protection, and particularly to methods and apparatuses that employ digital signal processing techniques. More particularly still, the preferred method and apparatus of the present invention are microprocessor based and utilize harmonic restraints and ground fault detection to provide reliable protection.
(ii) Description of the Prior Art
A general microprocessor based transformer monitoring system is disclosed in U.S. Pat. No. 4,654,806 granted Mar. 31, 1987 to Thomas D. Poyser et al. The system provides continuous on-line monitoring and analysis of transformer operation by monitoring various parameters related to transformer load (i.e. currents) and condition. Maximum, minimum, and instantaneous values of the parameters are stored and analyzed. To perform the analysis, a hierarchy of thresholds is associated with each parameter. When a parameter exceeds any one of the thresholds, a response is produced by the transformer monitoring system. The type of response depends on the level of the exceeded threshold in the hierarchy. The range of response produced by the transformer monitoring system includes: continuing normal periodic data collection and analysis, increasing the rate of data collection and analysis, recommending an on-site physical check of the monitored transformer, reducing transformer load, and taking the transformer off line.
A microprocessor based method for remote digital protection of distribution transformers is disclosed in U.S. Pat. No. 4,745,512 granted May 17, 1988 to John T. Hampson. The method measures and compares increases in both negative and positive phase reference currents on a distribution cable feeding a transformer and to trigger a circuit breaker to isolate the cable only if the negative sequence current increase exceeds a predetermined proportion of any simultaneous positive sequence current increase. Consequently, energizing with a 12.5% unbalance current has been found possible without false tripping. The supply parameters are sampled with a sampling period of one cycle of the supply waveform.
Yet a third computer based system is disclosed in U.S. Pat. No. 4,772,978 granted Sep. 20, 1988 to Yoshifumi Oura et al. In this transformer protection system, data of voltages and currents detected at individual terminals of a transformer connected to an electric power system are supplied to a computer. The computer computes driving point admittances of the transformer on the basis of the voltages and current data and predetermined transfer admittances of the transformer and decides that an internal fault has occurred in the transformer when the values of the driving point admittances or shunt admittances deviate from pre-set reference values, thereby disconnecting the transformer from the electric power system. Thus, in such a system physical constants of known values peculiar to the transformer being protected must be used in the computation.
The use of the second harmonic to distinguish the normal in-rush magnetizing currents from fault currents requiring protective action has been disclosed by several United States patents.
U.S. Pat. No. 2,290,101 granted Jul. 14, 1942 to Heinz Gutmann simply prevents operation of the protective differential relay from tripping in over-current cases where the second harmonic exists.
In U.S. Pat. No. 3,223,889 granted Dec. 14, 1965 to Edmund O. Schweitzer, in addition to the mere existence of the second harmonic also the phase difference between the harmonic and the fundamental must exceed a predetermined value for the protective relay to trip.
U.S. Pat. No. 4,477,854 granted Oct. 16, 1984 to Masaji Usui et al discloses a relay adapted for protection of a transformer and capable of functioning with certainty upon occurrence of any internal fault in the transformer but not functioning in the case of an inrush current which flows therein at the time of non-load energization or the like. The relay comprises a ratio differential element for comparing the amount of a suppression current with that of a differential current flowing in the transformer, an element for detecting the content proportion of a second harmonic component in the differential current, a monostable multivibrator for sending an output of a fixed pulse width at the moment of detection of the differential current, an AND circuit for producing an output in accordance with the logical product condition relative to the output of the second-harmonic detection element and that of the monostable multivibrator, a timing circuit for producing an output when the output of the second-harmonic detection element continues for a predetermined period of time, and gate means for suppressing the output of the ratio differential element when either the output of the AND circuit or that of the timing circuit is being fed thereto.
In U.S. Pat. No. 3,337,772 granted Aug. 22, 1967 to Stig Andersson, in addition to the second harmonic the fifth harmonic is used to stabilize the protection device against increase in the difference current caused by over-voltage, which causes an increase in the difference current of the fifth harmonic.
In U.S. Pat. No. 4,661,877 granted Apr. 28, 1987 to Masaji Usui, a protective relay for a transformer issues a relay tripping command when the differential current that is the difference between currents in the primary and secondary windings of the transformer has a magnitude larger that the specified value and, at the same time, when the differential current includes the fifth harmonic component less than the specified value. Another cause of tripping signal generation is the differential current value processed to have a certain timer characteristics.